In general, a construction machine such as a hydraulic excavator is formed of an automotive lower traveling structure, an upper revolving structure that is rotatably mounted on the lower traveling structure through a revolving device, and a working mechanism provided on the upper revolving structure. The upper revolving structure includes a revolving frame rotatably supported on the lower traveling structure, a cab provided on the front side of the revolving frame, an engine mounted on the rear side of the revolving frame, a counterweight located on the rear side of the engine and mounted on a rear end portion of the revolving frame in order to keep a weight balance with the working mechanism and the like.
The revolving frame to become the basis of the upper revolving structure includes a flat-plate shaped bottom plate on which the revolving device is mounted, left and right vertical plates extending in the front and rear direction on the bottom plate and installed upright in the left and right direction at an interval and on which a foot portion of a boom constituting the working mechanism is mounted, and a front plate located on the front side of the bottom plate and connecting the vertical plates to each other.
Here, the front plate is installed upright on the bottom plate so as to be inclined toward the rear side and fixed to each of the left and right vertical plates at the both end portions. A cylinder mounting bracket for mounting a boom cylinder for driving a boom is fixed on the front surface side of the front plate. Moreover, the front plate connects the left and right vertical plates to each other and is also formed as a reinforcing member for bearing application (a load) of excavating reaction force transmitted from the working mechanism along with each of the vertical plates and the like (Patent Document 1).